Ge quantum dots light-emitting devices

Abstract

Si photonics becomes one of the research focuses in the field of photonics. Si-based light-emitting devices are one of the most important devices in this field. In this paper, we review the Si-based light-emitting devices fabricated by embedding Ge self-assembled quantum dots into optical microcavities. Ge self-assembled quantum dots emit light in the telecommunication wavelength range from 1.3 to 1.6 μm, for which Si is transparent. Ge self-assembled quantum dots were grown on silicon-on-insulator (SOI) by molecular beam epitaxy (MBE) in Stranski-Krastanov (S-K) mode. Then, electron beam lithography (EBL) was used to define the pattern of optical microcavities on the wafer. Finally, the pattern was transferred onto the Si/Ge slab by inductive coupled plasma (ICP) dry etching. Room-temperature photoluminescence (PL) was used to characterize the light-emitting properties of fabricated devices. The results showed that strong resonant light emission was observed in different optical microcavities. Significant enhancement of the intensity was obtained by the optical resonance. Based on the results of PL, we designed and fabricated current-injected light-emitting devices based on Ge self-assembled quantum dots in optical microcavities. Room-temperature resonant light emission was observed from Ge dots in a 3.8 μm microdisk resonator.

Keywords

Si-based light-emitting devices / Ge self-assembled quantum dots / microcavities / photonic crystal (PhC) / microdisk

1 Introduction

In the past 10 years, the power conversion efficiency (PCE) of organic–inorganic hybrid perovskite solar cells (PSCs) has skyrocketed from 3.8 to 25.2% [1,2] because of their excellent optoelectronic properties, including high absorption coefficient [3,4], high carrier mobility [5], long balanced carrier diffusion length [6], and low exciton binding energy [7]. However, organic–inorganic hybrid PSCs face poor thermal stability caused by the organic compound [8]. The severe stability issue of PSCs could be effectively released when an inorganic cation, such as Cs⁺, partially or fully substitutes the organic cations [9,10].

For the stability issue of the all-inorganic PSCs, apart from the intrinsic phase stability, the selection of hole-transport material (HTM) has also shown a significant effect [11]. The most commonly employed HTMs in inorganic PSCs are still 2,2′,7,7′-tetrakis(N,N-dipmethoxyphenylamine)9,9′-spirobifluorene (Spiro-OMeTAD) and poly[bis(4-phenyl) (2,4,6-trimethylphenyl)amine] (PTAA) [12–21]. You and colleagues simultaneously prepared a CsPbI₃ device with the indium tin oxide (ITO)/SnO₂/CsPbI₃/Spiro-OMeTAD/Au structure in a completely dry nitrogen environment and obtained 15.7% PCE [12]. Zhao et al. fabricated a perovskite device with the ITO/c-TiO₂/CsPbI₃/Spiro-OMeTAD/Ag structure and gained an excellent performance of 19.09% [13]. Meanwhile, Liu et al. applied PTAA as the HTM for the CsPbI₃ device and achieved 15.07% PCE [14]. In these hole-transport materials, the dopant bis(trifluoromethane) sulfonamide lithium salt and 4-tertbutylpyridine, which promote conductivity and proton transfer, are easily precipitated when the device suffers from heating [22–24]. Therefore, exploring a dopant-free hole-transport layer is critical for delivering efficient and stable inorganic perovskite solar cells.

Poly(3-hexylthiophene-2,5-diyl) (P3HT) is a very conventional polymer with a very high mobility (10⁻³ cm²/(V∙s)). It has been successfully used in organic–inorganic hybrid solar cells [25]. In this work, we applied P3HT in all-inorganic PSCs with the ITO/SnO₂/LiF/CsPbI_3−xBr_x/P3HT/Au structure. As a result, the all-inorganic CsPbI_3−xBr_x PSCs achieved an excellent performance of 15.84%. In addition, the P3HT HTM-based device exhibited good photo-stability, maintaining ~80% of its initial PCE over 280 h under one Sun irradiation. The device also showed better thermal stability compared to the traditional hole-transport material, Spiro-OMeTAD.

2 Experiment

2.1 Materials

The SnO₂ nanoparticle precursor was purchased from Alfa Aesar (tin (IV) oxide, 15% in H₂O colloidal dispersion). Lead iodide (PbI₂), lead bromide (PbBr₂), N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), and chlorobenzene were obtained from Sigma-Aldrich. Spiro-OMeTAD was purchased from Xi’an Polymer Light Technology Corp. (China). Cesium iodide (CsI) was obtained from Aladdin Industrial, Corp. (China). P3HT was purchased from Solarmer (China).

2.2 Device fabrication

An ITO glass was cleaned by detergent, deionized (DI) water, acetone, and isopropanol in turn before drying in a N₂ flow. Next, the ITO substrate was cleaned by ultraviolet-ozone (UVO) treatment for 20 min, coated with the SnO₂ nanoparticle precursor at a speed of 3000 r/min for 30 s, and annealed at 150°C for 30 min in ambient air. Afterwards, the SnO₂/ITO substrate was disposed by UVO for 10 min. The perovskite film was prepared by the common one-step method. CsI (1 mol/L), HPbI₃ (0.68 mol/L), and PbBr₂ (0.32 mol/L) were then dissolved in a DMF–DMSO mixture (1:1.4 volume ratio) and stirred for 2 h. The PSC precursor was spun on the SnO₂ layer at 2600 r/min for 60 s, let stand by at room temperature for 1 h, and annealed at 170°C in ambient air for 10 min with humidity<40%. Subsequently, 30 mL P3HT solution with 10 mg/mL in chlorobenzene was spun on PSC 3000 r/min for 30 s and annealed at 100°C for 1 h. Finally, the electrode Au was coated by thermal evaporation with 80 nm thickness.

2.3 Device characterization

The photocurrent density–voltage (J–V) characteristic of the perovskite solar cell was finished by a Keithley 2400 ion source meter under one solar (AM 1.5 G) illumination using a solar simulator (EnliTech, Taiwan, China), and the KG-5 Si photodiode was used to calibrate the light intensity of the solar simulator. The photovoltaic cells were measured in a nitrogen glove box with reverse (1.2 V → 0 V, step 0.02 V) and forward (0 V → 1.2 V, step 0.02 V) scans. The scan electron microscopy image included the morphology and the cross-sectional structure of the device. Ultraviolet light was measured by a Varian Cary 5000 spectrophotometer. The photoluminescence (PL) spectrum was recorded using a FLS980 spectrometer (UK). The time-resolved PL spectra were measured by a F900 spectrometer (UK). The external quantum efficiency (EQE) curve was measured using the Taiwan EnliTech EQE measurement system (China). For the light stability test, unpackaged PSCs were immersed in a nitrogen glove box and in a continuous illumination using a white LED lamp (AM 1.5G, 100 mW/cm²).

3 Results and discussion

We prepared the all-inorganic CsPbI_3−xBr_x solar cell with the ITO/SnO₂/LiF/CsPbI_3−xBr_x/P3HT/Au structure as shown in Fig. 1(a). A thin layer of LiF was vacuum-evaporated on the SnO₂ surface to modify the interface between the electron transport layer (ETL) and the perovskite. The function of the LiF can be found in our recent report [26]. A CsPbI_3−xBr_x layer was fabricated by CsI, HPbI_3+x, and PbBr₂, applying the one-step spin-coating method. A black film was obtained after annealing under 170°C. A dopant-free P3HT was spin-coated on perovskite for the HTM. Figure 1(b) shows the band alignment and the charge transport mechanism.

Fig.1 (a) Device structure of the CsPbI 3−xBrx PSCs based on P3HT. (b) Energy band diagram of CsPbI3−xBrx PSCs based on P3HT. (c) Steady-state PL spectra of CsPbI3−xBrx perovskite (PSK), PSK/Spiro-OMeTAD, and PSK/P3HT prepared on glass. (d) Time-resolved PL spectra of CsPbI3−xBrx PSK, PSK/Spiro-OMeTAD, and PSK/P3HT prepared on glass

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We studied the charge transport properties of P3HT with the perovskite layer using PL. Compared with the perovskite itself and Spiro-OMeTAD depositing, the perovskite film showed a significant PL quench while depositing P3HT on the perovskite surface, indicating that the P3HT has good hole charge extraction for the perovskite layer that is even better than that of Spiro-OMeTAD. The PL emission for the perovskite and perovskite/Spiro-OMeTAD samples was located at 706 nm, while that for the perovskite/P3HT blue shifted to 700 nm, inferring that the P3HT could show a passivation effect while depositing on the perovskite surface [27]. This result could be mainly attributed to the sulfur (S) atoms from P3HT found bonding with the Cs and Pb atoms in the interface (Cs–S: 4.03 Å; Pb–S: 3.36 Å) and suppressing the anti-site defects on the CsPbI_3−xBr_x surface [28]. Figure 1(d) shows the corresponding time-resolved PL spectrum. The charge lifetime of the perovskite film with P3HT was much shorter than that with the perovskite and perovskite/Spiro-OMeTAD samples. The lifetimes of CsPb_3−xBr_x/P3HT, CsPb_3−xBr_x/Spiro-OMeTAD, and CsPb_3−xBr_x were 0.77, 1.50, and 1.93 ns, respectively. The results can further clarify the good carrier extraction at the perovskite/P3HT interface.

Figure 2(a) depicts the representative photocurrent density–voltage (J–V) curves of the PSCs based on the P3HT hole-transport layer. The champion cell achieved 15.84% PCE, with the short circuit current density J_SC of 18.43 mA/cm², the open circuit voltage V_OC of 1.12 V, and the fill factor FF of 76.96% measured by reverse scan. While the Spiro-OMeTAD-based device showed a slightly lower performance, the P3HT-based device exhibited a better performance that could be attributed to the perovskite surface passivation of P3HT. Figure 2(b) shows stable reverse and forward scans of 15.28% and 13.13%, respectively. Figure 2(c) depicts the typical external quantum efficiency. The integrated J_SC was 17.85 mA/cm², which closely matched with the measured J_SC from the solar simulator. Figure 2(d) illustrates the performance of the P3HT-based devices with high reproducibility. The average efficiencies of the 40 P3HT- and Spiro-OMeTAD-based devices were approximately 13.5% and 11.3%, respectively.

Fig.2 (a)  J–V curve of the inorganic CsPbI3−xBrx PSCs based on different HTMs. (b) Reverse- and forward-scan J–V curve from the inorganic CsPbI3−xBrx PSCs using P3HT as the hole-transport layer. (c) Typical EQE for the devices using P3HT as the hole-transport layer. (d) Efficiency histogram of the inorganic CsPbI3−xBrx PSCs using different HTM

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The ideal factor of the diode is a determinant parameter related to the interfacial defect recombination. The associated formula is nKT/q, where n is the ideal factor, K is the Boltzmann constant, T is the temperature, and q denotes the elementary charge. An n value closer to 1 indicates a lower density of the interfacial traps [29]. We tested the devices’ responses under different light intensities to estimate the ideal factor. For comparison, Spiro-OMeTAD was also used as the HTM. Both P3HT- and Spiro-OMeTAD-based devices revealed a linear relationship between the J_SC and the light intensity in Fig. 3(a), indicating no significant charge barrier at the interface. As shown in Fig. 3(b), the slope of V_OC to the light intensity for the P3HT-based device was 1.58KT/q, while that for the Spiro-OMeTAD-based device was 1.63KT/q. This result further confirmed that the P3HT layer can effectively decrease the surface defects at the perovskite/P3HT interface.

Fig.3 (a) Relationship between  JSC vs light intensity for devices based on different HTMs. (b) VOC versus light intensity for the inorganic CsPbI3−xBrx PSCs based on different HTMs

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We further investigated the thermal stability of the P3HT- and Spiro-OMeTAD-based devices by placing them on a hot plate at 85°C of continuous heating. We then examined the efficiency variation of the devices over time. The efficiency of the Spiro-OMeTAD-based device dropped dramatically within an hour, while that of the device with the P3HT HTM was maintained at 90% of the initial efficiency after 80 h heating (Fig. 4(a)). The photo-stability and the long-term storage stability of the device based on P3HT were also studied. The P3HT-based device can hold 80% of the initial efficiency under a continuous one Sun illumination (100 mW/cm²) after 300 h (Fig. 4(b)) and drop less than 10% when stored in N₂ glovebox even after 60 d (Fig. 4(c)). These results showed that the P3HT polymer HTM can improve the thermal stability of all-inorganic perovskite devices compared with the Spiro-OMeTAD. Moreover, the P3HT-based all-inorganic CsPbI_3−xBr_x solar cells also showed excellent photo-stability and long-term storage stability. The poor thermal or photo-stability of the Spiro-OMeTAD-based device could be caused by the dopant aggregation or the crystallinity of the Spiro-OMeTAD, which led to poor conductivity or pinholes in the Spiro-OMeTAD and resulted in bad contact or serious recombination. Meanwhile, for the P3HT layer, P3HT was well crystallized, can tolerate a temperature higher than 100°C, and will be very robust under thermal or photo stress [30–32].

Fig.4 (a) Thermal stability of CsPbI 3−xBrx PSCs under 85°C in N2 glove box. (b) Photo-stability of CsPbI3−xBrx PSCs under continuous white light LED illumination (100 mW/cm2) in N2 glove box. (c) Long-term PCE stability of a CsPbI3−xBrx PSCs without encapsulations stored in a dry glove box and tested once a week

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4 Conclusions

This study used the dopant-free polymer, P3HT, as the hole-transport layer for inorganic perovskite solar cells. The device showed 15.84% PCE, thereby presenting the best performance among all-inorganic PSCs based on the P3HT HTM. The most important thing herein is that the device can reserve the original efficiency by more than 90% while being heated up to 80 h at 85°C. The efficiency also maintained the initial 85% when irradiated under simulated 1.5 AM sunlight for 300 h.

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Acknowledgements

The authors would like to thank Prof. Usami from Tohoku University for his help in the growth of Ge quantum dots. This work was supported by the Fundamental Research Funds for the Central Universities of Huazhong University of Science and Technology (No. 2011TS022) and the National Natural Science Foundation of China (Grant No. 61177049).

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